Presently, data communications devices ("DCDs") and other data communications equipment ("DCE"), such as analog and digital modems, terminal adapters and routers, for example, are used to transfer or transport data between data terminal equipment ("DTE") such as personal computers, workstations, mainframe computers, and other terminals, over various communications channels such as telephone lines, T1 or ISDN lines, which also may be part of a computer network. Typically, the data is transmitted, received, or otherwise transferred in the form of a digitally encoded communication signal, which may include, for example, digitally encoded data transmitted on a carrier signal, having a predetermined or otherwise specified center (carrier) frequency, that is modulated by a predetermined constellation (or transmit constellation) of signal points (such as quadrature amplitude modulation), at a particular (and usually predetermined) signaling or symbol rate (also referred to as a baud rate). In current and proposed systems, the signaling constellation may include from five hundred to over 1600 signaling points. For a given channel, the data throughput will vary as a function of both the symbol rate and the carrier frequency.
At the sending or transmitting DCE, a carrier signal is thus modulated with the set of constellation signal points corresponding to the digital code or value to be transported over the channel. The channel, however, typically introduces or allows a variety of impairments or noise which affect the transmitted signal, such as amplitude and phase distortion, envelope delay distortion, non-linear distortion, additive noise, white noise, and other distortions. The level of distortion in the channel may limit the rate of data transport, as certain data rates, such as the 28.8 kbps available under International Telecommunications Union (ITU) Recommendation V.34, require reasonably high signal to distortion (noise) levels. Before the digital data is transported, therefore, a symbol rate and carrier frequency should be chosen which optimizes or maximizes the data throughput for a given channel.
With the advent of the V.34 protocol, multiple symbol rates and carrier frequencies are now available for selection in data transfer, in contrast with the prior art which largely relied upon fixed symbol rates and carrier frequencies. In addition, different channel characteristics may be more or less significant for the determination and optimization of a data rate which may be supported by a selected symbol rate and carrier frequency under the V.34 protocol. Accordingly, a need has remained for an apparatus, software modem, and method to rapidly and efficiently determine a symbol rate and carrier frequency for data transmission, to optimize data throughput for given channel conditions or characteristics, such as envelope delay distortion characteristics, and also for a given type of channel, such as ADPCM or those containing PCM codecs.